Embodiments herein generally relate to electrostatographic printers and copiers or reproduction machines, and more particularly, concern a printing method that constantly monitors the reload function of a developer donor roll to avoid the formation of ghost images on the printed product.
Some systems use a two component magnetic brush to load toner onto a donor roll, which delivers the toner to the image on the photoreceptor. After the toner is stripped from the donor rolls and delivered to the image, the donor roll reloads toner from the magnetic brush. However, the properties of the toner (mass, charge, size) in these reloaded areas are different from the non-reloaded areas. This leads to an image defect in the form of a ghost of the previous image at a distance of one or more donor revolutions. This image quality artifact is commonly referred to as the “Reload Defect.” Reload defects are also observed in single component development systems.
Development hardware and materials are optimized to address reload. Reload efficiency is a strong function of toner supply to the donor loading nip. The toner supply is increased by increasing the speed of the magnetic roll and increasing the developer packing fraction in the donor loading nip. However, both approaches increase the rate of developer material abuse and impact the overall developer material life. It has been observed that rotating the donor rolls in a direction against the magnetic roll improves reload. However, it has also been observed that the opposite surface movement mode loading leads to higher levels of mottle on the prints (lack of smoothness in halftone areas). In fact, most of the counter measures for good reload (opposite surface movement mode loading, high toner concentration (TC), conductive developer, alternating current (AC) bias in the loading nip) result in higher levels of mottle on the prints. Reducing reload using other means can open opportunities for optimizing the developer system design and material design to address image noise and life.
Reload efficiency is a function of the developer material state and environment and can vary over time and from one print job to the next. Reload efficiency can vary in the cross process direction due to inboard to outboard variation in TC, developer material flow and developer gap.
Exemplary embodiments herein create a latent printing image charge (that could be part of a print job) on a photoreceptor printing region of a photoreceptor within a printing apparatus. Either as a calibration operation, or simultaneously with the print job, the embodiments herein charge latent images of source patches on a printing region of the photoreceptor in the case of a calibration operation or a non-printing region of the photoreceptor outside the photoreceptor printing region in the case of a continuous monitoring operation.
The embodiments herein transfer marking material (e.g, toner, ink, etc.) from a donor roll to the photoreceptor by rotating the donor roll as the photoreceptor passes by the donor roll. The source patches cause marking material to be removed from areas of the donor roll to create marking material-depleted regions corresponding to the source patches. The marking material-depleted regions are reloaded with marking material using a magnetic brush.
After reloading the marking material-depleted regions with marking material (and simultaneously with the continuous creation of the printing image charge in continuous monitoring mode) the embodiments herein charge latent images of reload target patches and ideal target patches on the non-printing region of the photoreceptor. The reload target patches are located one donor roll rotation distance (equal to the circumference of the donor roll) on the photoreceptor from the source patches. The ideal target patches are located on the photoreceptor between the reload target patches. The reload (depleted) target patches and ideal target patches should be of the same area coverage (or color) so that any differences between them are only due the reload function.
The embodiments herein continue to transfer the marking material from the donor roll to the photoreceptor. This continuing process transfers the marking material to the reload target patches and the ideal target patches on the photoreceptor. Note that because the marking material-depleted regions were previously reloaded and because the reload target patches are spaced one donor roll rotation distance from the source patches, the reload target patches draw marking material that has been reloaded on the marking material-depleted regions of the donor roll. To the contrary, the ideal target patches draw marking material from regions of the donor roll between the marking material-depleted regions and, therefore, draw marking material from regions of the donor roll that have passed by the magnetic brush multiple times. The ideal target patches therefore draw marking material from regions of the donor roll that could be considered to be fully reloaded (or ideally reloaded).
After transferring the marking material to the reload target patches and the ideal target patches on the photoreceptor, the embodiments herein evaluate the reload function of the donor roll and the magnetic brush by comparing characteristics of the marking material on the reload target patches with marking material on the ideal target patches. This allows the embodiments herein to alter the printing image charge to maintain the reload function within a predetermined range.
This method constantly monitors the reload function of the developer material donor roll while the print job is printing to avoid the formation of ghost images on the printed product. Initial evaluation of the reload function may occur when the printing apparatus cycles up to print a print job. However, the subsequent “evaluation” and “correction” of the of the reload function and the “altering” of the printing image charge occur simultaneously with the printing apparatus printing one or more print jobs, and the evaluating and the altering processes avoids interrupting the print jobs.
The predetermined range of the reload function prevents ghost images from being perceptible within printed sheets produced by the printing apparatus. The method alters the printing charge to maintain the reload function within the predetermined range, yet avoids altering the relative rotational speeds of the donor roll and the magnetic brush, the packing fraction of developer material, and/or the developer material concentration. In other words, in some embodiments, the method alters only the printing charge to maintain the reload function within the predetermined range, which is substantially more efficient than methods that alter such aspects regarding the physical loading of developer material on the donor roll.
These and other features are described in, or are apparent from, the following detailed description.